Re: [REGRESSION] RLIMIT_DATA crashes named

[Laura Abbott <labbott@redhat.com>]

On 09/16/2016 10:46 AM, Linus Torvalds wrote:
> On Fri, Sep 16, 2016 at 8:16 AM, Laura Abbott <labbott@redhat.com> wrote:
>>
>> Fedora received a bug report[1] after pushing 4.7.2 that named
>> was segfaulting with named-chroot. With some help (thank you
>> tibbs!), it was noted that on older kernels named was spitting
>> out
>>
>> mmap: named (671): VmData 27566080 exceed data ulimit 23068672.
>> Will be forbidden soon.
>>
>> and with f4fcd55841fc ("mm: enable RLIMIT_DATA by default with
>> workaround for valgrind") it now spits out
>>
>> mmap: named (593): VmData 27566080 exceed data ulimit 20971520.
>> Update limits or use boot option ignore_rlimit_data.
>
> Ok, we can certainly revert, but before we do that I'd like to
> understand a few more things.
>
> For example, where the data limit came from, and how likely this is to
> hit others that have a much harder time fixing it. Adding Sam
> Varshavchik and Brent to the participants list...
>
> In particular, this is clearly trivially fixable as noted by Brent in
> that bugzilla entry:
>
>   'remove the "datasize 20M;" directive in named.conf'
>
> along with the (much worse) option of "use boot option
> ignore_rlimit_data" that the kernel dmesg itself suggests as an
> option.
>
> So for example, if that "datasize 20M;" is coming from just the Fedora
> named package, it would be much nicer to just get that fixed instead.
> Because RLIMIT_DATA the old way was just meaningless noise.
>

As far as I can tell this isn't Fedora specific.

> We definitely don't want to break peoples existing setups, but as this
> is *so* easy to fix in other ways (even at runtime without even
> updating a kernel), and since this commit is already four months old
> by now with this single bugzilla being the only report since then that
> I'm aware of, my reaction is just that there are better ways to fix it
> than reverting a commit that can be worked around trivially.

I was debating the merits of a revert. My concern is that this bugzilla
just represents the people who are reporting the bug and able to
correlate it to named. The actual number of people who are seeing
problems may be higher and anyone mucking with their config
could hit this and then have to go through troubleshooting steps again.
Add a config, get a segfault is a pretty terrible experience even
by Linux standards. I'd feel better about not reverting if there
were a proposed patch for named

I would like to see RLIMIT_DATA actually do something useful so worse
case I'll figure out something to carry in Fedora and this thread
can be an FYI for people googling.

>
>                  Linus
>

Thanks,
Laura

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